Multiplexer and low pass filter for multiplexer

ABSTRACT

Disclosed is a low-pass filter connected to a capacitor by a T-junction and used for a multiplexer. The low-pass filter includes: a plurality of capacitor elements which is formed of a capacitive conductor of a disk shape; and a plurality of inductor elements which connects the capacitor elements to each other and is formed in a rod shape. The plurality of capacitor elements and the plurality of inductor elements are alternately arranged, and the capacitor element which is connected to the capacitor has a capacitance value smaller than a capacitance value which is determined by a cut-off frequency of the low-pass filter.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 15/548,205 filed on Aug. 2, 2017, which is a National StagePatent Application of PCT International Patent Application No.PCT/KR2016/000915 filed on Jan. 28, 2016 under 35 U.S.C. § 371, whichclaims priority to Korean Patent Application No. 10-2015-0018141 filedon Feb. 5, 2015, which are all hereby incorporated by reference in theirentirety.

BACKGROUND

An exemplary embodiment of the present invention relates to themultiplexer and the low-pass filter for the same.

A multiplexer is a device for speeding up a data communication bymultiplexing several data transmission lines. It is a combinationalcircuit that connects a communication system between a base station anda user terminal by connecting one selected input line among severalinput lines to single output line, or a data received through singleinput line to several output lines. It may be referred to simply “MUX”.In addition, it is also referred to ‘Data Selector’ as it is possible tomake a single output with multiple-input data.

Conventionally, STDM (Statistical Time Division Multiplexer) supportingfrom 19.2 kbps to 56 kbps has been widely used to transmit data but hasdisadvantage that voice and data cannot be implemented together. Itmakes T1 grade MUX became generalized which has not only securehigh-speed transmission section but also mixed operate after combiningvarious communication networks like phone, fax or video conferencingsystem to data communication network.

The multiplexer processes a wide range of frequency band therefor, theinterference between the low frequency band and high frequency band canoccur when the difference between the low frequency band and highfrequency band is double. It make multiplexer hard to processlow-frequency band because of harmonic generation therefrom.

SUMMARY

The purpose of the present invention provides a multiplexer and alow-pass filter which can remove the harmonic by minimizing the bandinterference when processing a wide range of frequency bands.

According to an exemplary embodiment of the present invention, themultiplexer through which different frequency bands pass may comprise ahousing that includes an I/O terminal and a channel group which inputsand/or outputs frequencies of different ranges, includes a plurality ofconnectors, and is separated from the I/O terminal, a low-pass filterprovided inside the housing, electrically connected to the I/O terminal,and formed of a distributed constant type, a common capacitor providedin parallel to the low-pass filter and electrically connected to acontact point between the I/O terminal and the low-pass filter, and acavity filter which includes a plurality of cavities which are formed inthe housing and a resonator which is respectively installed in thecavities, wherein a part of the cavity filter is electrically connectedbetween the low-pass filter and a part of the channel group, and whereinthe other part of the cavity filter is electrically connected betweenthe common capacitor and the other part of the channel group.

In an exemplary embodiment, the cavity filter includes the cavitieswhich are formed by forming grooves from an upper portion toward a lowerportion, wherein the plurality of cavities are arranged in a pluralityof rows and a plurality of columns, and wherein the plurality ofcavities on both ends of the plurality of rows or the plurality ofcolumns among the plurality of cavities form a plurality of connectionpassages so as to be connected to each other corresponding to signaltransmission paths.

In an exemplary embodiment, each of the plurality of connection passagesincludes a transmission line for signal transmission between thepluralities of cavities which are located on both ends.

In an exemplary embodiment, the frequency band which passes through thelow-pass filter is lower than the frequency band which passes throughthe common capacitor.

In an exemplary embodiment, the low-pass filter includes a plurality ofcapacitor elements which is formed of a capacitive conductor of a diskshape and a plurality of inductor elements which is formed in a rodshape, wherein the plurality of capacitor elements and the plurality ofinductor elements are alternately arranged.

In an exemplary embodiment, one end and the other end of the low-passfilter are configured with the plurality of capacitor elements.

In an exemplary embodiment, a capacitance value of the capacitor elementon the one end or the other end of the plurality of capacitor elementsis smaller than capacitance values of the other capacitor elements byforming the capacitor element on the one end or the other end of theplurality of capacitor elements differently in size, diameter, or shapefrom the other capacitor elements.

In an exemplary embodiment, the capacitor element on the one end or theother end of the plurality of capacitor elements has a slope, which hasa protrusion in the center, on one side.

In an exemplary embodiment, the common capacitor includes a coaxialline, an external conductor which is separated from an outer perimeterof the coaxial line through a space, and a dielectric material whichfills the space.

In an exemplary embodiment, the dielectric material is formed of Teflon.

According to the exemplary embodiment of present invention, a low-passfilter connected to a capacitor by a T-junction and used for amultiplexer may comprise a plurality of capacitor elements which isformed of a capacitive conductor of a disk shape and a plurality ofinductor elements which connects the capacitor elements to each otherand is formed in a rod shape, wherein the plurality of capacitorelements and the plurality of inductor elements are alternatelyarranged, and wherein the capacitor element which is connected to thecapacitor has a capacitance value smaller than a capacitance value whichis determined by a cut-off frequency of the low-pass filter.

In an exemplary embodiment, one end and the other end of the low-passfilter are configured with the capacitor elements.

In an exemplary embodiment, a capacitance value of the capacitor elementon the one end or the other end of the plurality of capacitor elementsis smaller than capacitance values of the other capacitor elements byforming the capacitor element on the one end or the other end of theplurality of capacitor elements differently in size, diameter, or shapefrom the other capacitor elements.

In an exemplary embodiment, the capacitor element on the one end or theother end of the plurality of capacitor elements has a slope, which hasa protrusion in the center, on one side.

According to an exemplary embodiment of the present invention,separating the low frequency band and high frequency band to minimizeinterference between two frequency bands can remove the harmonic thatpossibly generated in low frequency band when low and high frequencybands are processed. The multiplexer accomplishes well phase matchingwhen low-pass filter connects to common capacitor with T-junction. Asusing the single multiplexer as a parameter, an implementation of abroader frequency band communication is possible to make simplificationand streamlining of the devices and equipment and lead to reduction ofcosts consequently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the schematic block diagram showing a multiplexer according toan exemplary embodiment of the present invention.

FIG. 2 is the diagram showing a low-pass filter according to anexemplary embodiment of the present invention.

FIG. 3 is the diagram showing a low-pass filter in accordance withanother exemplary embodiment of the present invention.

FIG. 4 is the schematic block diagram showing a common capacitoraccording to an exemplary embodiment of the present invention.

FIGS. 5A and 5B are blueprints showing a multiplexer according to anexemplary embodiment of the present invention.

FIG. 6 is a perspective view of a multiplexer according to an exemplaryembodiment of the present invention.

DETAILED DESCRIPTION

The present invention can be applied to various modification and have anumber of exemplary embodiments; therefore, some of specific embodimentswill now be explained in detail with the blueprints. However, this isnot intended to limit the invention to the specific embodiment but mayinclude all of the modifications, equivalents and substitutes in wholescope of idea and technology of the present invention.

In the following description of the present invention, if a detaileddescription of a related art is regarded as unnecessarily obscuring thesubject matter of the present invention, the detailed description may beomitted. Moreover, the number used in the description (e.g., the firstsecond, and so on) may be an identification symbol to distinguish theone component to another component.

In addition, the mention of one component “is connected to” the othercomponent or “is link to” may indicate directly connection or link butit also may be connected or linked through the intermediary of anothercomponent in the middle as long as the specifically opposite case is notmentioned.

Hereinafter, a specific content for the practice of the invention willbe described with reference to the accompanying drawings

FIG. 1 is a brief diagram showing a multiplexer according to anexemplary embodiment of the present invention.

Referring FIG. 1, the multiplexer 100 according to an exemplaryembodiment of the present invention includes a housing 110, a singleinput/output (I/O) terminal 120, a channel group 130 comprised of morethan one connector 131 and 132, a low-pass filter 140, a commoncapacitor 150, and a filter cavity 160.

The single I/O terminal 120 and the channel group 130 can be installedin mutually separated space in housing 110. The single I/O terminal 120and the channel group 130, each of which can be input or outputfrequencies. Herein, the channel group 130 can be composed each of thelow-frequency connector 131 and the high frequency connector 132. Theconnectors 131 and 132 may be provided at least one of each channelgroup and each of the connectors 131 and 132 may be input or output adifferent frequency bands. For example, the low-frequency connector 131may be input or output of the band 700 MHz to 1000 MHz and the highfrequency connector 132 may be input or output the band 1700 MHz to 2600MHz.

The low-pass filter 140 is installed on one side of the inner housing110 and may be electrically connected between the input and outputterminal 120 and the low-frequency connector 131. The low-pass filter140 composed with a plurality of capacitor element and a plurality ofinductor element that are disposed alternately to each other may beformed in a distributed integer line. Herein, the capacitor elementcomprising of a disc-shaped capacitive conductors may operate as a lowimpedance section. The inductor element comprising of rod shape mayoperate as a high-impedance section while connects the capacitorelement.

In addition, the frequency band passing through the low-pass filter 140can pass a lower frequency band than the frequency band through thecommon capacitor 150, for example, the 700 MHz 1000 MHz band. Moreover,the frequency band passing through the common capacitor 150 can pass ahigher frequency band than the frequency band passing through thelow-pass filter 140, for example, 1700 MHz to 2600 MHz band.

As described above, the multiplexer 100 in accordance with an exemplaryembodiment of the present invention makes low frequency band passedthrough the low-pass filter 140 while the high frequency band passedthrough the common capacitor 150. Therefore, it minimizes theinterference between the high and low frequency band and remove harmonicin the low-frequency band caused by interference thereof.

In other words, the multiplexer 100 may pass a broader range offrequency bands. For example, as described above, the multiplexer 100may pass the low frequency band of 700 MHz to 1000 MHz band and the highfrequency band of 1700 MHz to 2600 MHz band. Between the low frequencyband and the high frequency band passing through the multiplexer 100 islikely to cause interference. In particular, if the difference offrequency between them is two times, the harmonic may generate in lowfrequency band due to significant interference. In one example, one ofthe high frequency bands, 1800 MHz is different in 2 times as one of lowfrequency bands, 900 MHz among the frequency bands which the multiplexer100 processing. Therefore, the harmonic generation is the problem in thecase of 900 MHz frequency band passing through the multiplexer 100.

The multiplexer 100 according to one embodiment of the present inventionmakes the low frequency band pass through a distributed integer typelow-pass filter 140 while the high frequency band pass through thecommon capacitor 150 when the frequency differences with each other ismore than twice, therefore, it can suppress the generation of harmonicsin the low frequency band by minimizing the interference between the lowand high frequency band.

FIG. 2 is a view showing the low-pass filter 200 according to anembodiment of the present invention. Referring FIG. 2, the low-passfilter 200 may be formed of the distributed integer type filter. Morespecifically, the low-pass filter 200 may include a plurality ofcapacitor elements of a cylindrical shape 210, 220, and 240 and aplurality of rod-shaped inductor element 230. A plurality of capacitorelements 210, 220, and 240 and the inductor element 230 is disposedalternately with each other and the capacitor elements 210 and 240 maybe placed at the one end and the other one of the low-pass filter 200.The protrusions 211 can be formed at the capacitor element 210 and 240disposed at the one end and the other end of the low-pass filter 200.The protrusion 211 can be electrically connected with the single I/Oterminals 120.

Each of the capacitor elements 210, 220, and 240 may be formed with acapacitive conductor in a disk shape. The inductor elements 230 areformed in small rods shape whose diameter is smaller than the diameterof the capacitor elements 210, 220, and 240, and connect the capacitorelement 210, 220, and 240. Since the impedance value of the capacitorelements 210, 220, and 240 is smaller than the inductor elements 230,the capacitor elements in the distributed integer low-pass filter 200,210, 220, and 240 operates as a low impedance while the inductorelements 230 may operate as a high-impedance portion.

Even though the low-pass filter 200 of the distributed integer typelow-pass filter shown in FIG. 2 is comprised of 5 capacitor elements210, 220 a, 220 b, 220 c, and 240 and 4 inductor elements 230 a, 230 b,230 c, and 230 d, the present invention is not limited above. The sizeand number of the capacitor elements and the inductor elements can beset depending on the cut-off frequency of the low-pass filter 200 ofdistributed integer type.

In other words, the distributed integer type low-pass filter 140determines total number of the capacitor element and the inductorelement and the capacitance value of capacitor element and inductancevalue of inductor elements depending on the cut-off frequency. Forexample, when the cut-off frequency is 1.4 GHz in the distributedinteger low-pass filter 140, the distributed integer type low-passfilter 200 shown in FIG. 2 may have 5 capacitor element 210, 220 a, 220b, 220 c, and 240 and 4 inductor elements 230 a, 230 b, 230 c, and 230d. In the case, the capacitance value of the capacitor element 210, 220a, 220 b, 220 c, and 240 is determined respectively and the inductancevalue of the inductor element 230 a, 230 b, 230 c, and 230 d is alsorespectively determined theoretically. As a consequence, diameters andheights of the respective capacitor elements 210, 220 a, 220 b, 220 c,and 240 and diameters and lengths of the respective inductor elements230 a, 230 b, 230 c, and 230 d can be determined.

TABLE 1 0.01 db ripple n^(th) gn degree g1 g2 g3 g4 1^(st) 0.0960 1.0000degree 2^(nd) 0.4488 0.4077 1.1007 degree 3^(rd) 0.6291 0.9702 0.362911.0000 degree

[Table 1] shows the prototype (gn) value corresponding to the eachdegree of the distributed integer type low-pass filter 140, in the caseof setting frequency of bandpass of the distributed integer typelow-pass filter 140 has a ripple of 0.01 db as the value of theprototype g0 of the distributed integer type low-pass filter 140 sets 1when the cut-off frequency of the distributed integer low-pass filter140 sets 1. Calculation method of the value of the prototype (gn) isgeneral information; therefore a detailed description is omitted.

As shown in [Table 1], when the cut-off frequency of the distributedinteger low-pass filter 140, a prototype value, and the frequency ofbandpass of the distributed integer low-pass filter 140 is given, by thecorresponding to the each degree, the value of the inductance andcapacitance of the inductor element 230 a, 230 b, 230 c, and 230 d ofthe capacitor element 210, 220, and 240 can be determined. Therefore,the distributed integer low-pass filter 140 can be designed using thecapacitor element set 210, 220, and 240 and the inductor elements 230 a,230 b, 230 c, and 230 d.

The low-pass filter 140 may be connected to the common capacitor 150with the T-Junction when the distributed integer type low-pass filter140 is employed by the multiplexer 100 according to an embodiment of thepresent invention, That is, the one end of the capacitor elements of thelow-pass filter 140 and 210 may be connected to the common capacitor 150with T-junction and it may be connected to the single I/O terminal 120electrically. When the low-pass filter 140 and the common capacitor 150are linked, the one end of the capacitor elements of the low-pass filter140 and 210 and a common capacitor 150 is connected. Thereafter, aproblem is occurred that the phase matching between the capacitorelement 210 and the common capacitor 150 cannot be achieved.

According to an embodiment of the present invention, the low-pass filter140 achieves a phase matching between the capacitor element 210 and thecommon capacitor 150 by making smaller the capacitance value of thecapacitor element 210, which is located in the one end to the low-passfilter 140 connecting the common capacitor 150 with T-junction than thetheoretical value of the capacitor element 210. More specifically, asdescribed above, the total number of the capacitor elements and theinductor elements and respectively corresponding capacitance values andinductance values are theoretically determined according to the cut-offfrequency in the distributed integer type low-pass filter 140. When thecapacitor element located in the one end of the distributed integer typelow-pass filter is connected to the common capacitor with T-junction, itcannot achieve a phase matching. However, the phase matching is wellachieved if the capacitor value of the capacitor element located in theone end of the distributed integer type low-pass filter which isconnected the common capacitor with T-junction is smaller than thetheoretical capacitance value which described above.

According to an exemplary embodiment, to making smaller a capacitancevalue of a capacitance element 210 which is located in the one end ofthe low-pass filter 200 connection with a common capacitor withT-junction than the theoretical capacitance value, the size or diameterof the capacitance element located in the one end of the distributedinteger type low-pass filter 210 may be formed smaller than the size ordiameter of the capacitance which is theoretically determined. Thecapacitor elements and the inductor elements are arranged symmetricallyin the distributed integer type low-pass filter which is thermionicallydetermined. For example, in case of having 5 capacitor elements as inFIG. 2, around the capacitor element 220 b located at the center of 5capacitor elements, the inductor elements 230 b and 230 c which isadjacent to the capacitor 220 b have same inductance value and theinductor elements which is located outside the inductor elements 230 band 230 c also have equal value each other.

However, in theory, if the same size of the capacitor elements 210 and240 have the same capacitance value, the capacitor element 210 connectedto a common capacitor 150 with T-junction cannot achieved the phasematching. Therefore, the capacitor element 210 connected to the commoncapacitor 150 with T-junction has a smaller capacitance value formed bymaking smaller in height or diameter than the capacitor element 240,which has a theoretical capacitance value for the phase matching.

The capacitance value of the capacitor element 210 can be made smallerthan thermionically determined capacitance value by forming smallerdiameter or the height of the capacitor element 210 which located in theone end of the low-pass filter connected to common capacitor withT-junction, than one of other capacitor elements 240. In this way, thephase matching can be easily achieved.

FIG. 3 is a view showing a low-pass filter 300 in accordance withanother embodiment of the present invention. Capacitor elements 310 and330 may be disposed in the one end and the other end in the low-passfilter 300. Wherein one capacitor element 310 disposed in the one end ofthe low-pass filter 300 has a protruding portion 311 forming in thecenter of the capacitor element 310 which may be electrically connectedto the single I/O terminal 120

The other capacitor elements except the one end 310 and the other end340 may be disposed in a symmetrical shape around the center of thelow-pass filter 300.

Each of the capacitor elements 310, 320, and 340 may operate as a lowimpedance portion by forming capacitive conductor in disk shape. Inaddition, an inductor element 330 may operate in a high-impedanceportion by forming in rods shape connecting the capacitor element 310,320, and 340.

The capacitor element 310 located in the one end of the low-pass filter300 is connected to the common capacitor 150 with T-junction, wherebythe outer surface is formed to have a slope is smaller than thetheoretical capacitance value, as described above, the phase matchingwith the common capacitor 150 may be able to achieve easily. The commoncapacitor 150 installed in parallel with the low-pass filter 140, iselectrically connected to between the contact point 141 of the singleI/O terminal 120 and the low-pass filter 140 and the high frequency bandchannel group 132. As an example, the one end of the common capacitor150 may connect the capacitor element 210 located in the one end oflow-pass filter 140 to the single I/O terminal by T-junction. The otherend of the common capacitor 150 may be connected with the filters 162which process the high frequency band.

The higher frequency band than the frequency band to pass through thelow-pass filter 140, for example, the frequency band of 1700 MHz to 2600MHz is inputted through the high frequency channel group 132 and thefrequency bands pass through the filter 162 connected to of respectivechannel group 132 and it can be transmitted to the single I/O terminal120 through the common capacitor 150.

FIG. 4 is a schematic view showing the common capacitor 150, accordingto an embodiment of the present invention. Referring to FIG. 4, thecommon capacitor 150 may be formed of a coaxial line 151, the outerconductor 152 having a separated space from a perimeter of coaxial line151, and dielectric 153 filled the separated space. The dielectric 153may be formed of Teflon.

The common capacitor 150 can pass the higher frequency band than thefrequency band passing through the low-pass filter 140. For example, theband passing through the low-pass filter 140 is 700 MHz-1000 MHz rangeand the band passing through a common capacitor 150 may be 1700 MHz-2600MHz range. As described above, by passing the high frequency bandthrough the common capacitor 150 and the low frequency band through thelow-pass filter 140, the interference between the high frequency bandand a low frequency band and may be minimized. Therefore, the harmonicthat may occur in the low-frequency band caused by the interferencementioned above can be removed.

A conventional multiplexer may occur an interference between the lowfrequency and high frequency because the frequency of the differentbands implemented within one device. As a result, A conventionalmultiplexer have had a limit of input and output frequency band becausea harmonic may be generated when low frequency pass the multiplexer dueto the interference.

However, the multiplexer 100 according to an embodiment of the presentinvention uses the low-pass filter 140 and the common capacitor 150, asdescribed above. A low frequency band, for example, 700 MHz-1000 MHzrange of the frequency band passes through the low-pass filter 140 and ahigh frequency band, for example, 1700 MHz-2600 MHz range of thefrequency band passes through the common capacitor 150. Therefore, theharmonic generation when low frequency band passed through themultiplexer 100 can be eliminated.

In addition, the low-pass filter 140 and the common capacitor 150 areconnected by T-junction when the multiplexer 100 is implemented. In thiscase, the low-pass filter 140 and the common capacitor 150 havepre-determined capacitance value in accordance with the cut-offfrequency. If those values are used, a phase matching is notaccomplished properly at the contact point 141 of the low-pass filter140 and the common capacitor 150. At this time, to accomplish a properphase matching, the multiplexer 100 according to an embodiment of thepresent invention make smaller, a capacitance value of the one end ofcapacitor element 210, 310 which are disposed in the low-pass filter 140than the capacitance value that is pre-determined a capacitor valueaccording to the cut-off frequency, as an example the size or diameterof one end of capacitor elements 210, 310 make smaller than the othercapacitor element of the symmetry, or the capacitor elements 210, 310can be formed differently from the capacitor element of the symmetry.

This multiplexer 100 according to an embodiment of the present inventionincludes wider range of a frequency band than a conventionalmultiplexer, in particular, if difference between high and lowfrequencies are double, it can pass through a low frequency band andhigh frequency band, whereby it can achieve a cost reduction due tosimplification and streamlining of the equipment constituting thecommunication system.

A filter 161 disposed between the low-pass filter 140 and thelow-frequency channel groups 131 and other filter 162 disposed betweenthe common capacitor 150 and the high frequency channel groups 132, maybe the cavity filter 160.

The cavity filter 160 forms the housing 110 including a plurality ofcavities therein, and a resonators are provided in each of the cavities,whose parts of them provides electrical connection between the low-passfilter 140 and low-frequency channel groups 131, and the others mayprovide electrical connection between the common capacitor 150 and thehigh frequency channel groups 132.

The cavity filter 160 may form a groove toward the lower direction fromthe upper and equipped to cavities. The cavities may be disposed to forma plurality of rows and a plurality of columns. The cavities locatedopposite ends of rows and column among mentioned cavities may form aplurality of connection passages connected to each other in response tothe signal transmission path.

Wherein the transmission line for signal transmission between thecavities located at the opposite ends of the connection passage may beinserted and installed within each of the connection passage.

FIGS. 5A and 5B are blueprints showing a multiplexer 400 in accordancewith another embodiment of the present invention using a cavity filter.FIG. 6 is a perspective view of the multiplexer 400 shown in FIGS. 5Aand 5B. More specifically, FIG. 5A is a plan view of the multiplexer 400in accordance with one embodiment of the present invention, FIG. 5B is abottom view of the multiplexer 400 in accordance with one embodiment ofthe present invention. Referring FIGS. 5A and 5B, the multiplexer 400comprises a housing 410, a single I/O terminal 420 separated by thehousing 410, a channel group 430 and 440 comprised by connecters whichinput/output different band of frequencies 432, 433, 434, 441, 442, 443,and 444, a cavity filter 470, a low-pass filter 450, and a commoncapacitor 460.

Referring FIG. 6, the multiplexer 400 comprises the housing 410, thesingle I/O terminal 420 separated by the housing 410, the channel group430 and 440 comprised by connecters which input/output different band offrequencies 432, 433, 434, 441, 442, 443, and 444, the cavity filter470, the low-pass filter 450, and the common capacitor (not shown here).

The cavity filter 470 is comprised of the cavities 471 which formed bycutting housing 410 perpendicularly from the outside to inside by thepredetermined depth and equipped with resonator 472 in the center of thecavity 471 toward the direction of the cavity formed. In addition, eachof the cavities 471 can be connected to adjacent one in the directionfrom the single I/O terminal 420 to the channel group 430 and 440

The part of the cavity filter 470 may connect electrically between thelow-pass filter 450 and low-frequency channel groups 430 and the otherpart of the cavity filter 470 connect electrically between the commoncapacitor 460 and the high frequency channel groups 440.

According to an exemplary embodiment of the present invention, themultiplexer 400 utilize the low-pass filter 450 in a low frequency bangand the common capacitors 460 in a high frequency band passing thefrequency, so that the multiplexer 400 may remove the harmonicgeneration when a low frequency band pass the multiplexer 400.

In addition, the low-pass filter 450 and the common capacitor 460 can beconnected with T-junction, in this case, the low-pass filter 450 and thecommon capacitor 460 have pre-determined capacitance values accordancewith the cut-off frequency. If those values are used, a phase matchingis not accomplished properly at the contact point of the low-pass filter450 and the common capacitor 460. At this time, to accomplish a properphase matching, the multiplexer 400 according to an embodiment of thepresent invention may make smaller capacitance value of the capacitorelements 210 and 310 which are disposed in the one end of low-passfilter 450 than other pre-determined capacitor value in accordance withthe cut-off frequency, as an example the size or diameter of one end ofthe capacitor elements 210 and 310 is made smaller than the othercapacitor element of the symmetry, or the capacitor elements 210 and 310may be formed differently from the capacitor element of the symmetry.

The multiplexer 400 according to an embodiment of the present inventionmay pass wider range of a frequency band than a conventionalmultiplexer, in particular, if difference between high and lowfrequencies are double, whereby it can achieve a cost reduction due tosimplification and streamlining of the equipment constituting thecommunication system.

Above explanation is merely shown the example of skilled art andtechnology of present invention. Those skilled in art of this field maybe able to make a variety of modifications and variations withoutdeparting from the scope of present invention.

Therefore, certain described embodiment of present invention is notintended to limit the art of skill but give an explanation and thatexample may not limit the art of skill and ideas.

The scope of the present invention shall be determined only according tothe attached claims and equivalent art of skill and ideas are also beincluded in the spirit and scope of these claims.

What is claimed is:
 1. A low-pass filter connected to a capacitor by aT-junction and used for a multiplexer, the low-pass filter comprising: aplurality of capacitor elements which is formed of a capacitiveconductor of a disk shape; and a plurality of inductor elements whichconnects the capacitor elements to each other and is formed in a rodshape, wherein the plurality of capacitor elements and the plurality ofinductor elements are alternately arranged, and wherein the capacitorelement which is connected to the capacitor has a capacitance valuesmaller than a capacitance value which is determined by a cut-offfrequency of the low-pass filter.
 2. The low-pass filter of claim 1,wherein one end and the other end of the low-pass filter are configuredwith the capacitor elements.
 3. The low-pass filter of claim 2, whereina capacitance value of the capacitor element on the one end or the otherend of the plurality of capacitor elements is smaller than capacitancevalues of the other capacitor elements by forming the capacitor elementon the one end or the other end of the plurality of capacitor elementsdifferently in size, diameter, or shape from the other capacitorelements.
 4. The low-pass filter of claim 3, wherein the capacitorelement on the one end or the other end of the plurality of capacitorelements has a slope, which has a protrusion in the center, on one side.